1. Field of the Invention
The present invention relates to a process for the production of a toner for electrophotography.
2. Description of the Prior Art
Toners used for electrophotography have been minimized in their particle size so as to meet the recent demand for high precision images, and is generally obtained as fine particles having an average particle size ranging 1 to 10 .mu.m. With respect to processes for the preparation of such particles, the so-called pulverization method in which resins, pigments, etc., are mechanically mixed and kneaded, and then pulverized is generally used. However, one of the problems with the pulverization method is that the smaller the particle size of a toner, the more complicated the facilities and processes become for obtaining a desired toner; this has made the production costs high. Another problem is that in terms of the pulverization characteristics, the fluidity is poor.
In contrast, the emulsion dispersing method has been disclosed as a process for the preparation of toners having a small particle size in Japanese Laid-Open Patent Applications No. Hei 3-15078, Hei 4-78863, Hei4-178654, etc. In this method, a polymer solution made by dissolving a polymer in a water-insoluble solvent is emulsified and dispersed in an aqueous dispersion solution so that an O/W type emulsion is formed, and the O/W emulsion was heated while being stirred so as to evaporate the organic solvent, thereby allowing polymer particles to precipitate.
In the emulsion dispersing method, processes are simplified and polymer fine particles are obtained through comparatively simple operations. As compared with the pulverization method, a wide range of resins can be used, and the application of the resulting polymers is widened. At present, in the case when polymer fine particles are obtained by the emulsion dispersing method, an emulsifying machine of a high-speed shearing system, such as T.K. AUTO HOMO MIXER (made by Tokushu Kika K.K.), is utilized in which stirring and mixing are carried out by shearing induced by high-speed revolutions of blades, etc.
However, emulsifying machines of this type are basically operated on a batch basis, resulting in a problem of production efficiency. Moreover, another problem arises due to their inherent mechanical construction wherein layers tend to be separated into two types--a layer subjected to shearing and a layer in convection, it is difficult to obtain a sharp particle size distribution.